1. Field Of The Invention
This invention relates to electrochemical reduction-oxidation reactions which occur in electrolytic solutions at electrodes comprising Magneli phase titanium oxide and an apparatus for performing such reactions. For ease of reference this class of reactions will be generally referred to as soluble "redox" reactions, that is, those reactions where both oxidized and reduced species are stable and/or soluble in the reaction solution. Such reactions may be contrasted to those where one of the oxidation or reduction products is either a solid or a gas which would immediately separate from the electrochemical solution in which it was formed.
Magneli phase titanium oxides are those of the general formula Ti.sub.x O.sub.2x-1, where x is a whole number 4-10. Such oxides have ceramic type material properties, but are nevertheless sufficiently conductive to be used as electrodes. Thus, electrodes formed from these oxides will sometimes be generally referred to herein as "ceramic" electrodes. The utility of these materials in electrochemical applications has only recently come to light, and their properties in particular instances are only now being investigated.
The present invention is specifically directed to redox reactions in which it is normally desired to obtain the most efficient electrochemical conversion of a less desirable soluble species to a more desirable oxidation or reduction reaction product in solution. Since electrochemical processes are electron transfer reactions that occur at the electrode, activity in the bulk of the electrolyte away from the electrodes is generally confined to migration to or from the electrodes and mixing of the species in the solution. The activity within a few molecular diameters of the electrodes is the area in which the electron transfer reactions take place. This interface area has been the subject of much study in an effort to modify the behavior of species in the solution so as to optimize the electrochemical process. The use of electrocatalytic coatings, enhanced turbulence, increased electrode surface area and other strategies have been applied with some success.
When such a means of enhancing the efficiency of a reaction has been identified then a strategy must be developed for minimizing the back reaction of the desired species to its original state. This is a natural problem, since the oxidation and reduction reactions occur virtually simultaneously at the opposing electrodes in an electrolytic solution. Approaches to this problem include the separation of the electrodes by use of a partitioned cell, i.e., one in which a membrane or diaphragm separates the anolyte from the catholyte. The use of a smaller electrode for the reaction at which the reversion, or back reaction, occurs is also known, so as to form a greater volume of the desired reaction product at the larger electrodes.
By identifying efficient electrode materials and the most appropriate electrochemical cell design for a given redox reaction, profitable industrial processes for the production of or recovery of valuable chemical constituents can be developed. Currently these processes are used for metal plating, metal recovery, electric storage batteries, electrowinning and fine chemical and dyestuff manufacture, among others.
2. Description of Related Art
The art of use of electrochemical redox reagents in electrochemical processing is very well documented. Early references go back over 80 years in European technical literature. The use of cerium sulfate and chromic acid as a `Sauerstoffubertrager` or oxygen carrier, dates back to patent DRP No. 172654 (1903) for the manufacture of organic quinones. In this process cerium salts were added to the electrolyte. It was realized that cerium ion could be oxidized at a lead dioxide anode. The oxidizing agent produced is then reacted with anthracene to form anthraquinone. Ceric ion is reduced to the cerous state to be reoxidized at the anode once more and so act as a shuttle species between the anode and the insoluble organic substrate.
Reference to the contemporary literature shows that the uses of redox reagents in electrochemical processes is quite extensive. See Indirect Electrochemical Processes, Clarke, R. L., Kuhn, A. T., Okoh, E. Chemistry in Britain 59, 1975, Mantell, C. L. Industrial Electrochemistry, McGraw-Hill, New York. Baizer, M. M. (1973) Organic Electrochemistry, Marcel Dekker, New York. Weinberg, N. L. (ed) (1975) Techniques of Chemistry, Vol. 5 techniques of Electroorganic Synthesis, Parts I and II, John Wiley and Sons, Chichester and New York.
Redox reagents have been used in organic reduction processes such as the use of small amounts of tin to improve the yield of para-amino phenol from nitrobenzene by reduction at a cathode. The oxidation of toluene to benzaldehyde with manganese III in strong acid, the manganese III ion is generated at the anode, from manganese sulfate the product of the toluene oxidation process. More recently iron redox has been used to oxidize coal and other carbonaceous fuels to carbon dioxide, water and humic acid, See Clarke R. L. Foller Journal of Applied Electrochemistry 18 (1988) 546-554 and cited references. In this study, ferric ion in sulfuric acid was used as the redox reagent to oxidize carbonaceous fuels such as coke. In the process ferric ion was reduced to ferrous which is easily reoxidized to ferric at the anode. This ferrous to ferric oxidation occurs at potentials well below the oxygen evolution potential of the anode and is thus energy saving with respect to its use in the formation of hydrogen from water.
The presence of redox reagents in an electrochemical process is not always beneficial. In the electrochemical recovery of silver from photographic solutions, iron in the solution interferes with the cathodic deposition of the silver. Ferric ion competes with silver for electrons at cathode and is preferentially reduced to ferrous ion, such that the presence of small quantities of iron will reduce the efficiency for silver deposition below 20%.
The use of specific redox reagents in electrochemical reactions both as aids, or as the principle reactant is well understood by those skilled in the art. The present invention, however, concerns the use of specific electrodes to manipulate the redox effect to great advantage, that is, to be able to manipulate the choice of electrode material to promote a particular redox effect and/or reduce the effect at the counter electrode.
Electrode materials have usually been chosen from a group of metals such as platinum, nickel, copper, lead, mercury and cadmium. Additional choices might include irridium oxide and lead dioxide. The choice of electrode material is predicated on its survival in a particular electrolyte, and the effect achieved with the reagents involved. For example, to oxidize cerium III ion a high oxygen overpotential electrode is usually chosen such as lead dioxide. Some electrode materials are unable to oxidize cerium which requires an electrode potential of 1.6 volts as the oxygen overpotential of the metal electrode is too low, examples would be platinum and carbon. To reduce many organic substrates lead electrodes are chosen which has a very high hydrogen overpotential. Low hydrogen overvoltage electrodes such as platinum, nickel, iron, copper, etc. allow the hydrogen recombination reaction at the surface to occur at potentials too low to be effective as reducing cathodes for many organic substrates.
More recently conductive ceramics for use in certain electrochemical applications have been described. U.S. Pat. No. 4,422,917 describes the manufacture of Magneli phase titanium oxides and suggests the use of these materials in electrodes for certain electrochemical applications. This patent describes the properties and method of manufacture of a group of substoichiometric titanium oxides of the formula TiO.sub.x, where x ranges from 1.67 to 1.9. More specifically, it is taught at column 13, lines 27 to 32 that anodes of such titanium oxides coated with specified metals "may be satisfactory for use in redox reactions such as the oxidation of manganese, cerium, chromium and for use as products in the oxidation of organic intermediates."
In addition to the art describing efficient electrode materials, many publications describe electrochemical cell designs which seek to minimize redox back reactions and therefore optimize a process using an electrode efficient for a particular reaction.
Many examples of specific cell designs are to be found in the literature which attempt to reduce the back reaction. Robertson et al, Electrochimica Acta, vol. 26, No. 7, pp. 941-949, 1981, describe a cell system in which a porous membrane is used to cover the cathode of a hypochlorite generator to reduce the reduction of hypochlorite at the cathode to chloride. This same system was used to oxidize manganese to manganate and cerous to ceric. The system works by inhibiting the mixing of the bulk of the electrolyte at the electrode interface. A porous felt cover would allow escape of hydrogen into the electrolyte, and a concentration gradient would be set up with respect to the products of oxidation in the bulk of the electrolyte compared to access to the cathode. Alternatively, the cell can be designed with a small counter electrode with respect to the anode or viceversa. An example of this is described in Industrial Electrochemistry (1982) D. Pletcher, Chapman Hall, New York. See pages 145-151. Other descriptions of cell design strategies are to be found in Electrochemical Reactor Design (1977) D. J. picket, Elsevier, Amsterdam, and Emerging Opportunities for Electro-organic processes (1984), Marcel Decker, New York.
The fundamental method of dealing with back reactions is to operate a divided cell system, by inserting a membrane or diaphragm between the anode and cathode. The problem with this strategy is the cost of the electrochemical cell and its supporting equipment is much higher than in the case of an undivided cell. Further the cell voltage is higher due to the increased IR drop through the electrolyte and membrane, which also increases operating costs.
Thus, even the higher efficiency cell designs have their drawbacks. Complicated cell designs require a greater number of components, and this may become very expensive on an industrial scale. Systems which use a large electrode opposing a smaller electrode are undesirable since high voltages are required.
For these reasons a need has arisen for a redox system wherein an efficient electrode can be used, but which does not require a complicated cell design to prohibit the shuttling of the desired chemical species from the electrode at which they are formed to the opposing electrode to be reconverted to their original form.